The present invention relates to an optical lens system which is provided with a light source and a micro-lens array in which micro-lenses to which the light is projected from the light source are provided in a two-dimensional manner and an image display apparatus using the optical lens system.
In general, according to an image display apparatus having an image display device of non-emission-type, the transmittance (or the reflectance) of the image display device is changed in accordance with a driving signal (image signal) so that the intensity of the light that is projected from a light source to the image display device is modulated. The modulation allows the image and/or the characters to be displayed.
As such an image display apparatus, well known are (a) a direct-vision mode image display apparatus in which the image and/or the characters on the image display device are subjected to the direct-vision and (b) a projection mode image display apparatus in which the image and/or the characters on the image display device that have been enlarged and projected by a projection lens onto a screen are seen.
Further, as an image display device for use in the image display apparatus, listed are a liquid crystal display device, an electro chromic device, an EL (electro luminescence) device, a PDP (plasma display panel), and other devices. Among them, the liquid crystal display device has been widely used for devices such as a monitor, a projector, a portable information terminal, and a portable telephone.
In the liquid crystal display device, driving voltages are applied to respective pixel electrodes provided regularly in a matrix manner in accordance with image signal so that the optical property of the liquid crystal is changed. The changing of the optical property causes the displaying of the image and the characters.
As the way to apply the driving voltages to the respective pixel electrodes independently, well known are a simple matrix method and an active matrix method.
Among the methods, in the active matrix method, (a) switching devices such as MIM (metal-insulator-metal) devices acting as the non-linear two-terminal device and TFTs (thin film transistors) acting as the three-terminal device and (b) wiring electrodes for supplying the pixel electrodes with the driving voltages should be provided with respect to the liquid crystal display device.
The strong light incident on the device causes the device resistor in an OFF-state to be reduced so that the electric charges that have been charged during the voltage application are discharged and the proper voltage is not applied to a liquid crystal part that is located in a region where the switching device and the wiring electrode are provided. Under the circumstances, the following problem is raised. More specifically, since the display operation of the main body is not carried out, the leakage of the light occurs even in a black displaying state so as to reduce the contrast ratio.
In the case where the liquid crystal display device is of a transmission-type, in order to cut off the light directed to the region where the light that have earlier described should not be projected, it is necessary to provide light shield means referred to as a black matrix 1502 (a) on a TFT substrate which is provided with the switching devices and the pixel electrodes and/or (b) on an opposite substrate which is opposite to the TFT substrate via a liquid crystal layer, as shown in FIG. 11, for example.
Thus, according to the liquid crystal display device of transmission-type, in addition to a TFT 1501 acting as switching device having a shielding property, gate and source bus lines 1503 and 1504 acting as the wiring electrode having a shielding property, the black matrix 1502 acting as the light shielding means are involved in the light shielding. This causes the area of the effective pixel aperture section occupying in the block of the pixel, i.e., the aperture ratio, to be reduced.
Further, it is difficult to provide the foregoing switching device and the wiring electrode so as to have a size of not more than a predetermined scale. This is because the switching device and the wiring electrode respectively have the constraints relating to the electric performance and the manufacturing technique. This causes that the aperture ratio is further reduced as the pitch of the pixel electrodes becomes smaller in accordance with the high precision and miniaturization of the liquid crystal display device.
When the aperture ratio thus becomes small, since the amount of the light that transmits the liquid crystal display device is reduced, the problem of the shortage of the brightness occurs especially when the liquid crystal projector in which the small liquid crystal display panel made of the liquid crystal display device is subjected to the enlargement and projection to a big screen is used as the projection mode image display apparatus.
In view of the problem, realized is a method in which micro-lenses are used so as to converge the light onto the respective pixel aperture sections and so as to improve the effective aperture ratio of the liquid crystal display device.
For example, in Japanese unexamined patent publication No. 4-60538 (publication date: Feb. 26, 1992), disclosed is a projector of single plate-type in which the white light is directed to dichroic mirrors provided in a fan-shaped manner so that the white light is separated into the light beams of the respective colors R(red), G(green), and B(blue) and is directed at respective different angles to the micro-lens provided on the side of the light source of the liquid crystal display device. This ensures to converge the light beams onto the pixels corresponding to the respective colors.
Most micro-lenses are provided in an arrayed manner on the opposite substrate in the liquid crystal display device. For example, as shown in FIG. 12, the micro-lenses are arranged so that they are sandwiched between two glass substrates and refract the light between the grass and the resin or between the two kinds of resins. This ensures the converging effect.
This kind of micro-lens array (MLA) is manufactured in accordance with the process shown in FIGS. 13(a) through 13(d) or shown in FIGS. 14(a) through 14(e), for example.
First, in FIG. 13(a), the patterning of a photoresist is developed on a glass substrate, and the heat dripping is made so as to obtain a lens shape on the surface of the glass substrate. Then, in FIG. 13(b), the photoresist is subjected to the dry etching so as to transfer the shape of the photoresist onto the surface of the glass substrate, thereby obtaining a micro-lens substrate.
Subsequently, as shown in FIG. 13(c), a bonding agent causes a cover glass to be bonded by to the surface on the side of the micro-lens substrate where the lens is formed, the micro-lens substrate being a resultant of the process shown in FIG. 13(b). And, as shown in FIG. 13(d), the surface of the cover glass is polished so as to have a predetermined thickness, so that a micro-lens array for use in the liquid crystal display device is manufactured. The process is disclosed in Japanese unexamined patent publication No. 6-250002 (publication date: Sep. 9, 1994).
Another manufacturing method is as follows: first, as shown in FIG. 14(a), the patterning of a photoresist is developed on the glass substrate so as to prepare a master.
Next, as shown in FIG. 14(b), a metal stamper is prepared by using the master prepared in the process shown in FIG. 14(a). And, the shape of the micro-lens array is copied to the glass substrate by the metal stamper. Thus, a micro-lens substrate is prepared.
Then, as shown in FIG. 14(d), a bonding agent causes a cover glass to be bonded to the surface on the side, where the lens is formed, of the micro-lens substrate that has been prepared in the process shown in FIG. 14(c). Thereafter, as shown in FIG. 14(e), the surface of the cover glass is polished so as to have a predetermined thickness. Thus, a micro-lens array for use in the liquid crystal display device is manufactured.
In a direct-vision mode image display apparatus, the foregoing micro-lens array is used for improving the directivity by refracting the light projected from a backlight source to the front direction as disclosed for example in Japanese unexamined patent publication No. 10-39118 (publication data: Feb. 13, 1998) that is hereinafter referred to as a publication 1. Alternatively, the foregoing micro-lens array is used for relieving the dependency of the viewing angle of the liquid crystal display device by scattering the light that has transmitted through the liquid crystal display device as disclosed for example in Japanese unexamined patent publication No. 9-49925 (publication data: Feb. 18, 1997) that is hereinafter referred to as a publication 2.
Note that the micro-lens array is provided on the light incident side of the liquid crystal display device in the publication 1, and the micro-lens array is provided on the light reflection side of the liquid crystal display device in the publication 2.
By the way, the lens in general converges the light by use of the refraction function exerted in the interface of the media whose indexes of refraction are different from each other.
More specifically, in the case where the light directs to the medium whose index of refraction is smaller from the medium whose index of refraction is greater, when the incident angle of the light is great with respect to the plane-normal of the interface of the two media, the light is reflected from the interface without transmission. In contrast, as shown in FIG. 15, in the case where the light directs to the medium whose index of refraction is greater from the medium whose index of refraction is smaller, even when the incident angle of the light is great with respect to the plane-normal of the interface of the two media, the light transmits the interface. In FIG. 15, n1 and n2 (n1 less than n2) indicate the indexes of refraction, respectively, xcex81 indicates an incident angle of the light directing from the medium whose index of refraction is n1 to the interface, xcex82 indicates an refracted angle at which the light incident on the interface at an angle of xcex81 is refracted in the medium whose index of refraction is n2. Note that n1xc3x97sin xcex81=n2xc3x97sin xcex82 is satisfied.
As has been described above, since most of the micro-lens arrays are arranged so that they are sandwiched between two glasses, the refraction of the light occurs in the interface between the glass and the resin or in the interface between the two resins.
When the micro-lens array is used in the liquid crystal projector acting as the projection mode image display apparatus, the light from the light source is converged onto the aperture section of the pixel so as to pass through the aperture section of the pixel, and is then diffused so that it is directed to the projection lens.
However, according to the conventional liquid crystal projector, since the value xe2x80x9cFxe2x80x9d of the projection lens is great (the receiving angle of the light is small), when the converging angle of the micro-lens array is great (the radius of curvature of the lens is small), the light, having a diffusion angle of greater than the receiving angle of the projection lens, among the light that has transmitted the aperture section of the pixel is cut off by the projection lens.
Accordingly, in the conventional liquid crystal projector, it is realized by the micro-lens that the total amount of the light that reaches the screen becomes the largest by considering the balance of the light that passes through the aperture section of the pixel and the light that is cut off by the projection lens. This results in that the radius of curvature of the respective lenses in the micro-lens array becomes relatively great and no total reflection in the periphery part of the lens occurs.
In order to reduce the light that is cut off by the projection lens, it is contrived to reduce the value xe2x80x9cFxe2x80x9d of the projection lens (enlarge the receiving angle of light) so as to enlarge the converging angle of the micro-lens. Namely, it is contrived to reduce the radius of curvature of the respective lenses of the micro-lens.
In contrast, in the foregoing liquid crystal projector, the quartz glass or xe2x80x9cneo ceramxe2x80x9d made by Nippon Electric Glass Co. Ltd., that has an index of refraction ranging from about 1.46 to 1.54, is used as the glass substrate used in the liquid crystal display device. The resin having an index of refraction ranging from about 1.38 to 1.6 is now available in general as the resin for bonding the micro-lens between the glass substrates.
When the indexes of refraction of the glass and resin and the radius of curvature of the lens are adjusted, it is possible to realize a micro-lens array having a target focal length.
The light is refracted at a greater angle in an interface between the two media as the difference of the indexes of refraction between the two media is greater. According to the combination of the glass and the resin, the difference of the indexes of refraction between them is about 0.2 at most. This makes it impossible for the lens to have an enough converging performance. In order for the lens to have an enough converging performance, it is necessary that the radius of curvature of the lens interface is small, i.e., the incident angle in the refraction interface is great.
When the radius of curvature of the lens of the micro-lens array is thus small, the incident angle of the light becomes greater in the periphery of the lens. And, when the incident angle is not less than a predetermined angle, the light reflection occurs in the periphery of the lens as has been described earlier. This causes that the light does not transmit the curved surface of the lens, so that the effect of the micro-lens is reduced. In this case, the condition that makes the light to reflect from the curved surface of the lens appears to satisfy the inequality: (n2/n1)xc3x97sin xcex8xe2x89xa71, where n1 and n2 (n1 less than n2) indicate the indexes of refraction of the media before and after the curved surface of the lens (see FIG. 16), respectively, and xcex8 indicates an incident angle of the light.
For example, in the case of considering a lens in which the light is refracted and converged in the interface between a glass substrate having an index of refraction n2=1.54 and a resin having an index of refraction n1=1.38 in a 0.9-inch XGA (extended graphics array) panel having a pixel pitch of 18 xcexcm, when the curved surface of the lens is not more than about 15 xcexcm, the light is reflected in the periphery of the lens, provided that the parallelism of the illumination light is xc2x110xc2x0. This causes the problem that the lens effect of the micro-lens is reduced.
Thus, when the above micro-lens array is used, the amount of the light that transmits the micro-lens array is reduced in the liquid crystal projector acting as the projection mode image display apparatus. This causes the problem that it is not possible to obtain enough brightness.
In the case where a liquid crystal display apparatus adopting, for example, a micro-lens array as the direct-vision mode image display apparatus is used for improving the directivity by refracting the light projected from a backlight source to the front direction, when a micro-lens array in which the light is reflected in the periphery of the lens is used, the problems arise that the loss of the light occurs and the effect that improves the light directivity is weakened.
In the case of a liquid crystal display apparatus in which a micro-lens array is used for relieving the dependency of the viewing angle of a liquid crystal display device by scattering the light that has transmitted through the liquid crystal display device, when a micro-lens array in which the light is reflected in the periphery of the lens is used, the problems arise that the loss of the light occurs and the effect that improves the characteristic of the viewing angle of the liquid crystal display device is weakened.
The light reflected by the micro-lens includes the light that is reflected again by the member such as the surface of a substrate constituting the micro-lens array and is directed to the aperture section of a pixel other than the target pixels of the liquid crystal display device.
For example, in a liquid crystal display apparatus in which a black-and-white panel is used as the liquid crystal display device and the light having corresponding colors is separated and directed to the aperture sections of the respective pixels so as to carry out the color display, the amount of the foregoing light that has been reflected in the periphery of the lens is small. However, this indicates that the reflected light is directed not only to the pixels of the corresponding colors of the liquid crystal display device but also to the pixels corresponding to different colors. This causes the problem that the color purity is reduced due to the color mixture.
The present invention is made in view of the foregoing problems, and its object is to provide an optical lens system, an image display apparatus, a micro-lens array, a liquid crystal display device, and a liquid crystal display apparatus of projection-type that (1) have a micro-lens array made of minute lenses (micro-lenses) each of which has a curved surface satisfying a condition which allows the light to reflect in the periphery of the lens and (2) eliminates the reflection of the light in the periphery of the micro-lens so as to increase the amount of the light which transmits the micro-lens array for improving the lens effect, thereby ensuring (a) to obtain a bright projection image in a liquid crystal projector acting as a projection mode image display apparatus using the micro-lens array, (b) to obtain a display image with high color purity and without color mixture, (c) to improve the directivity of the light in a liquid crystal display apparatus acting as a direct-vision mode image display apparatus, and (e) to relieve the dependency of the viewing angle.
In order to achieve the above object, in an optical lens system in accordance with the present invention which is provided with a light source and a micro-lens array in which micro-lenses to which the light from the light source is directed are provided in a two-dimensional manner, when the micro-lens has a curved surface satisfying an inequality (1) of (n2/n1)xc3x97sin(xcex8max)xe2x89xa71, the micro-lens array is provided so that the light from the light source is directed to the micro-lens from the side of the medium having the index of refraction of n1, where n1 indicates an index of refraction of a medium constituting one side of the curved surface of the lens, n2 (n1 less than n2) indicates an index of refraction of a medium constituting the other side of the curved surface of the lens, xcex8 indicates an incident angle of the light with respect to a plane-normal of the curved surface of the lens when the light from the light source is directed to the curved surface of the lens of the micro-lens from the side of the medium having the index of refraction of n2, and xcex8 max indicates a maximum value of the angle xcex8 in the curved surface of the micro-lens.
Since sin xcex8xe2x89xa61 is satisfied, the inequality (1) is satisfied when the light is directed from the medium having a greater index of refraction to the medium having a smaller index of refraction. In this case, when the light having the incident angle xcex8 which satisfies the inequality (1) is directed to the micro-lens, the light is reflected in the periphery that has a great incident angle xcex8.
In contrast, when the light is directed from the medium having a smaller index of refraction to the medium having a greater index of refraction, the term (n2/n1) becomes (n1/n2). This causes that no reflection occurs in the light refraction plane of the lens, even when the light incident angle becomes great.
This ensures to eliminate the reflection of the incident light in the periphery of the micro-lens. Accordingly, it is possible to increase the amount of the light projected from the micro-lens array.
Further, the arrangement may be as follows: More specifically, light separation means for separating the light of the light source into the respective light having wave lengths of red, green, and blue and for directing the respective light to the image display device at respective different angles is further included, the micro-lens array is provided between the light separation means and the image display device and is provided so that each of the micro-lenses constituting the micro-lens array corresponds to one of three pixel groups corresponding to the respective light having wave lengths of red, green, and blue.
With the arrangement, even in a projection mode image display apparatus adopting the optical lens system having the above arrangement, when the color image is subjected to the projection display, it is possible to obtain a bright color display image.
Accordingly, the arrangement may be as follows: More specifically, (a) the image display device, which modulates the light from the light source in accordance with the image signal, is provided on the side of a surface, from which the light is projected, of the micro-lens array in the optical lens system having the above arrangement, (b) the respective micro-lenses of the micro-lens array are provided so as to correspond to the respective pixels of the image display device, and (c) each micro-lens is provided so as to converge the light from the light source to a aperture section of its corresponding pixel.
Namely, the optical lens system having the above arrangement may be used in a liquid crystal projection as one of image display apparatuses of projection-type.
In this case, it is possible to converge the light from the light source to the pixel of the liquid crystal display device and to improve the effective aperture ratio, thereby ensuring to obtain the bright projection image.
Further, another micro-lens array in accordance with the present invention is provided with a micro-lens supporting substrate which has a plurality of micro-lens sections, on a first surface, each micro-lens section having a convex surface that becomes a micro-lens, and a cover glass bonded to the first surface of the micro-lens supporting substrate via a resin having an index of refraction which is smaller than that of the micro-lens section, the micro-lens supporting substrate being provided so that a second surface which is an opposite surface of the first surface is polished so as to have a predetermined thickness.
The thickness of the micro-lens supporting substrate is set so as to be equal to or less than a length between a surface of the lens and a point to which the light is converged by the micro-lens section.
A further micro-lens array in accordance with the present invention is arranged so as to satisfy an inequality (2) of sin(xcex8 max)xe2x89xa7(n4/n3), where n3 indicates an index of refraction of the micro-lens section, n4 indicates an index of refraction of the resin, xcex8 max indicates a maximum angle between a plane-normal of a second surface of the micro-lens supporting substrate and a normal of a convex surface of the micro-lens section.
In this case, when the micro-lens array is provided so as to satisfy the inequality (2), it is possible to eliminate the reflection of the incident light in the periphery of the micro-lens, as has been described earlier. Accordingly, it is possible to increase the amount of the light projected from the micro-lens array.
In the case of a direct-vision mode liquid crystal display apparatus in which the liquid crystal display device is provided between the micro-lens array having the above arrangement and the light source as the image display device, since it is possible to diffuse the light projected from the liquid crystal display device more widely, it is possible to eliminate the loss of the light and to enhance the effect that improves the dependency of the viewing angle of the liquid crystal display device.
In the case of a direct-vision mode image display apparatus in which the liquid crystal display device is provided on the side of a surface from which the light is projected in the micro-lens array having the above arrangement, since the diffused light from the light source is effectively converted into the parallel light, it is possible to eliminate the loss of the light and to improve the directivity of the light.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. The present invention will become more fully understood from the detailed description given hereinafter and the accompanying drawings that are given by way of illustration only, and thus, are not limitative of the present invention.